In general, a construction machine such as an excavator supplies working oil to an actuator such as a hydraulic cylinder or a hydraulic motor to drive a working device such as a boom, an arm, a bucket, or an upper swing body.
In this case, a driving direction of the working device is controlled by a main control valve (MCV). In more detail, the working oil discharged from the hydraulic pump is supplied to each actuator while a flow direction of the working oil is controlled through the main control valve converted according to a work signal of an operating unit such as a joystick or a pedal, and as a result, driving each actuator is controlled.
That is, a plurality of spools is provided in the interior of the main control valve and a plurality of actuators is connected to the exterior of the main control valve, and when a worker generates a request pressure value which is a flow control signal in the operating unit which is a flow request unit such as the joystick or the pedal, the request pressure value is provided to the main control valve and a pump control device and a specific spool is opened/closed by the request pressure value in the main control valve, and as a result, the working oil is provided to the actuator associated with the corresponding spool by the opening/closing operation of the corresponding spool.
Meanwhile, a boosting function in the construction machine such as the excavator as a function to increase thrust of a cylinder by increasing a pressure of a hydraulic system is used when large force is temporarily required during working.
For example, there is a case in which a large rock needs to be excavated and a lot of soil needs to be put in the bucket unexpectedly while excavating stones having a relatively small size in a quarry and in this case, a boosting button is pressed, which is provided in the joystick, and the like that is configured to implement the boosting function in order to temporarily exert the large force.
FIG. 1 is a diagram illustrating a hydraulic system for a construction machine having a boosting function in the related art. A method that implements the boosting function in the hydraulic system for a construction machine in the related art will be described with reference to FIG. 1.
Referring to FIG. 1, in the hydraulic system for a construction machine in the related art, two main pumps 10 are provided and the working oil discharged from the main pumps 10 is provided to a main control valve 20. When a specific spool is actuated in the main control valve 20, the working oil is provided to an actuator (not illustrated) associated with the corresponding spool, and as a result, the actuator that receives the working oil performs desired work.
In addition, a main relief valve 30 is provided between discharge lines 12 connecting the main pumps 10 and the main control valve 20 to each other and a shuttle valve 40 is connected to the discharge line 12 in parallel.
Herein, the main relief valve 30 in which a desired maximum pressure is set serves to stabilize a maximum pressure of the hydraulic system by discharging some of the working oil when the maximum pressure is formed in the working oil in the hydraulic system.
In the hydraulic system, when the worker operates a button attached to a joystick 50, and the like in order to implement the boosting function, a valve 70 connected to a pilot pump 60 is switched and opened and the pressure of the pilot pump 60 is input into the main relief valve 30 and the set pressure increases to increase the pressure of the entire hydraulic system, and as a result, thrust or torque of each actuator increases.
Meanwhile, in the hydraulic system of the traditional construction machine, energy loss of the working oil of approximately 55% occurs due to loss such as pressure loss, and the like while the working oil passes through the main control valve. Due to such a reason, in recent years, a system has been developed, which configures a closed circuit by deleting the main control valve and directly connecting the hydraulic pump to each of the plurality of actuators and controls a swash plate angle of the hydraulic pump to control the flow direction of the working oil provided to the actuator.
In the construction machine constituted by the closed circuit hydraulic system, the hydraulic pump is allocated to each actuator and for example, when a separate direction switch valve, and the like are not used, a total of 7 hydraulic pumps are provided in order to drive the boom, the arm, the bucket, the upper swing body, left and right carriages, and an option device.
However, in order to implement the boosting function in the construction machine constituted by the closed circuit hydraulic system, since the relief valves need to be used in all of respective pumps similarly to application to the hydraulic system in the related art and control signals for controlling the plurality of relief valves increase, excessive increase of parts and cost and complexity of the hydraulic system are caused.